1. Field of Invention
The present invention relates to wind turbine blades and more particularly to the production of a simple, stiff, lightweight filament-reinforced mandrel of nonuniform wall thickness for the spar of a wind turbine blade.
2. Description of the Prior Art
In the current press for new forms of energy sources, one attractive candidate has been the wind turbine. In general, a wind turbine comprises an arrangement of rotor blades, hub, pitch change mechanism, gear box, tower, generator and typically an electrical interface, all adapted to extract energy from atmospheric winds and convert it into electrical or other useful forms of energy.
The large wind turbine (100 KW and over) is characterized by its extremely long rotor blades (typically at least fifty feet long each although they may be as long as one hundred feet or more each) which are subjected to severe bending and twisting at design loadings. The rotor blades are advantageously constructed in the spar/shell configuration, with the spar being fabricated by filament winding on a mandrel, as indicated for example in U.S. Pat. No. 4,081,220 issued on Mar. 28, 1978 to Andrews and owned by assignee common to the present invention, which disclosure is incorporated herein by reference. While the nature of the spar mandrel is not discussed in detail in the aforementioned Andrews' patent, it typically consists of a central load carrying structure in the nature of a steel channel box member upon which are carried a series of separate removable sections consisting of formers, stringers and sheet metal skin which are held in place on the central box beam with keys, pins, or the like. To facilitate mandrel removal since there is considerable taper from the butt to the tip end of the spar and since the former/stringer/skin assembly is carried on the outer surface of the central box beam, the load carrying capacity at the tip of the mandrel is very low. The limitation on the thickness of the tip of the mandrel requires that it be supported primarily as a cantilevered beam with a very small portion of the weight (up to a maximum of approximately three percent) carried on a steady rest at the tip. This type of mandrel construction is extremely heavy (on the order of twenty tons for a one hundred foot blade spar) and imposes a substantial overhung moment on the headstock of the machine which supports and rotates the mandrel during winding, resulting in massive structure bearings and foundation requirements.
In addition to the weight, a mandrel constructed in several sections as heretofore described always has motions at the joints between the sections due to deflections in the structure as the mandrel rotates. These motions cause heavy wear on the mandrel sections and also result in distress in the composite material as it is being applied and cured.